In radiosurgery, brain tumors (and other real formations) are destroyed by an intense beam of radiation. A necrotic dose is delivered to a tumor by cross-firing from multiple directions, in order to reduce the amount of energy deposited in healthy tissue. Hence, unlike with more invasive surgery, tissue surrounding the tumor can be protected to some extent. Although radiosurgery has been in use for several years, high ablative accuracy has only recently been made possible by progress in focused radiation sources and imaging techniques. With a risk that is proportional to both dose and the volume irradiated, radiation necrosis of tissue adjacent to a treated lesion remains the major complication of stereotaxic radiosurgery. Concerns remain as to whether particular volumes of tissue receive too much or too little radiation according to the prescription for treatment.
The radiosurgical treatment consists-of several phases. First, a precise three-dimensional (3D) map of the anatomical structures in the area of interest (for example, the brain) is constructed using computed tomography (CT) and magnetic resonance (MR) techniques. Next, a motion path for the radiation beam is computed to deliver a dose distribution that the surgeon finds acceptable (taking into account a variety of medical constraints). Finally, a jointed mechanism moves the radiation source according to this path.
A collimated radiation source is positioned in a sequence calculated to localize the energy deposition into a volume that as closely as possible conforms to that requiring treatment, while avoiding exposure of nearby healthy tissue. A system and method for performing stereotaxic surgery is disclosed in U.S. Pat. No. 5,207,223 issued to Adler on May 4, 1993 and in U.S. patent application Ser. No. 07/989,045, which is a continuation-in-part application of issued U.S. Pat. No. 5,207,223 which are incorporated by reference herein. The forward dosimetry problem is to compute the dose distribution in a tissue given a treatment plan. The inverse dosimetry problem is to find a treatment plan whose execution will achieve a desired dose distribution. Issues related to planning are not discussed in the above discussed patent or patent application. The planning methods of the present invention rely on a system with general kinematics such as the Neurotron-1000 such which is described in the above mentioned patent and patent application.
The dose distribution is an important parameter is radiosurgery. Whether fixed or frameless stereotaxic radiosurgery is used, it is important to plan a particular scheme for the application of radiation beams to the tissue. By providing dose distribution within the prescribed limits, treatment would be optimized and damage to healthy tissue would be minimized. It is thus desirable to find a suitable motion for the beam given the shape and the location of the tumor.